How Many Fish You Can Keep in Your Aquarium?

It is a natural assumption that the larger the aquarium, the more fish you can keep in it. Whilst this is obviously true, it is a statistic that can only be applied proportionately depending on what area of fishkeeping interest you intend to follow. It all depends on the oxygen levels in the water. Tropical aquarium water temperatures mean less oxygen than in coldwater tanks, and larger coldwater fish use up oxygen more than their relatively small tropical counterparts. Again, salt water contains a different amount of oxygen than freshwater. These factors conspire against the assumed conclusion.

In any aquarium, regardless of its actual dimensions and volume, you should understand that the maximum number of fishes kept is in this descending order: tropical freshwater, coldwater freshwater, tropical marine. The following illustration shows the typical maximums for a 90 cm X 38 cm X 30 cm (30in. X 15in. X 12 in.) tank.

Applying the guides:
- Tropical (freshwater) fish 30cm2 per 1cm length of fish.
- Coldwater (freshwater) fish 60cm2 per 1 cm length of fish.
- Tropical (marine) fish 120cm2 per 1cm length of fish.

The comparative total fish stocks that can be held in each tank, depending on type of fish kept, are:
- Tropical (freshwater): 90 cm (36″) total fish length.
- Coldwater (freshwater): 45 cm (18″) total fish length.
- Tropical (marine): 22.5 cm (9″) total fish length.

Incidentally, when measuring a fish, only measure the standard length (SL) from snout tip to end of body – don’t include the tail.

For the newcomer to the hobby, there are excellent easy entry-level systems available from the aquatic dealer. Best described as ‘fill up and go’ aquariums, each comes complete with lighting and filtration systems and it really is a case of furnish the tank, fill up with water, plug into the electric supply and you’re up and running. Introducing the fish comes a little later once the tank has settled down.

Reading about the separate components – and their functions – that make up the whole aquarium form will not only give you a better understanding of how the aquarium actually works but it will also provide a firm basis upon which you may want to go on to plan your own, larger, aquarium in years to come.

There are also other factors to take into consideration when choosing an aquarium. Size, we have already dealt with. Most aquariums supplied for sale nowadays have to comply with certain construction standards and usually bear some kind of trade-agreed label or sticker to this effect. This is important and should not be disregarded.

Water exerts a tremendous pressure upon the glass walls of the tank. In long aquariums, the front and back panels can easily bow outwards under such pressure and to prevent this there are usually bracing straps across the top of the tank from front to back. As the volume of the tank increases, so should the thickness of the glass used in its construction; again agreed thickness recommendations are in place for aquariums made by reputable companies.

Small acrylic tanks are often aimed at the children’s market but these are usually too small for long-term fishkeeping success.

The choice between glass and acrylic is a personal one, the main perceived drawback being that acrylic scratches more easily during cleaning. However, providing the correct cleaning pad is used, there should be minimal damage caused and acrylic’s ‘formed-all-in-one-process’ does allow for some remarkable aquarium designs.

Although you should always choose the largest aquarium you can afford (for the sake of the fish, you understand), you should also ensure that it doesn’t take up every inch of its allocated space -make sure that there is enough room around, above and beneath the aquarium for regular, trouble-free maintenance tasks.

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